(a) Field of the Invention
The invention is directed to rechargeable or non-rechargeable lithium cells which operate in a non-aqueous medium at temperatures which do not generally exceed 175.degree. C. More specifically, the invention relates to the composition of soft composite anodes which supply lithium, and can be produced as thin films and comprises the following elements: a finely divided alloy or intermetallic compound of lithium, a plastic or elastomeric macromolecular material with ionic conduction and a finely divided carbon additive. The invention relates to the use of these anodes in electrochemical generators, which can be prepared as thin films.
The use of plastic or elastomeric polymers as electrolytes enables to eliminate some disadvantages of liquid electrolytes (convection, problems of imperviousness . . . ) while overcoming the major difficulty of solid electrolytes which are crystalline or glassy, i.e. loss of contact between the electrolyte and the materials of the electrodes resulting from variations in the volumes of the electrodes during the operation thereof.
(b) Description of Prior Art
The French Patent Applications of Michel Armand and Michel Duclot, Nos. 78.32976, 78.32977 and 78.32978, now published under Publication Nos. 2,442,512, 2,442,514 and 2,442,513, advocate the use of solid elastomeric materials with ionic conduction as electrolyte in an electrochemical generator whose anode may consist of an alkaline metal, an alloy and/or an insertion compound which can release an alkaline cation. Among the alkaline metals, lithium is particularly interesting.
The present invention intends to optimize the composition and the operation of composite anodes, which preferably work with lithium and utilize such types of polymeric complexes (solvating polymer plus lithium salt) as electrolytes.
The applicant has observed that the designing of generators with thin films of the order of 100 .mu.m per elementary cell presents some technical difficulties for the formation of metallic lithium as thin films and of a quality sufficient to be compatible with thin cells using polymers for the electrolytes. Moreover, it has been observed that for rechargable cells, the use of metallic lithium is limited because of the problems resulting from the formation of dendrites and the passivation of the interfaces. Efforts have therefore been directed towards high activity lithium metallic alloys, such as lithium-aluminium, lithium-silicon and lithium-bismuth.
These materials, which can be ground by known processes, possess many characteristics which make them particularly well suited to batteries operating with ionic polymers (thin films). It is indeed possible to prepare on a large scale composite materials (alloy in powder form plus electrolyte) for example from a suspension in an organic medium. These composite electrodes then have a large specific area which enables the use of electrode materials in which the diffusion coefficients of lithium are relatively low.
Moreover, the use of powders of metallic alloys has many advantages such as:
the possibility of using thin electrode materials (&lt;100 .mu.m) by a plurality of methods, such as: pressing, sprinkling, suspension, coating, etc . . . ; PA0 multiplying the real surface of the active material in contact with the electrolyte and the current collector. PA0 a plastic or elastomeric macromolecular material with ionic conduction; PA0 a finely divided alloy of lithium the granulometry of which is lower than 40 .mu.m and selected so that the activity of the lithium in said alloy corresponds to potentials lower than +1.2 volts with respect to a lithium electrode; PA0 particles of a carbon compound of the formula: Li.sub.x C where 0.ltoreq.x&lt;0.3, the particles of the carbon compound being finely divided to a granulometry lower than 40 .mu.m. PA0 X is a trivalent element which can have 4 coordinated valences, selected among boron or aluminium, PA0 the groups R are aprotic hydrocarbon radicals, PA0 X is a halogen; PA0 n varies between 1 and 4; PA0 Y is CO or SO.sub.2. PA0 lithium-aluminium; PA0 lithium-silicon; PA0 lithium-antimony; PA0 lithium-bismuth; PA0 lithium-boron; PA0 finely divided lithium. PA0 40%: TiS.sub.2 PA0 17%: graphite PA0 43%: PEO-LiClO.sub.4 complex in a ratio O/Li=8 PA0 pressing while hot at a temperature higher than the melting point of the polymer the various components in powder form; PA0 or preparation of a thin film anode by means of known methods of painting or printing by solubilizing or suspending the polymeric electrolyte, the carbon additive and the alloy powder in compatible solvents, e.g. benzene, THF, dioxolane, etc., and the evaporation and the drying of the films obtained.
The only disadvantage of using lithium alloys is the drop in the coefficient of activity of lithium in the alloy, which represents a decrease of the potential of the electrode of 160 mV for Li.sub.3 Si and of 900 mV for Li.sub.3 Sb with respect to metallic lithium.
Unfortunately, on the side of the anode, most of the powders of lithium alloys are very reactive and are very easily passivated in spite of all the precautions taken when handling them. The result is a multiplication of the resistive contacts which are inherent to the fine powders, which means a bad distribution of the potential in the mass of the material leading to a bad utilization of the lithium of the electrode of the order of 3%.
The description made by Armand in the French patent applications mentioned above, does not always permit to obtain an active material especially at the negative electrode. This is particularly true for the powders of negative electrodes where one constantly faces problems resulting from the use of lithium, which seem due on the one hand to the multiplication of the resisting contacts which are inherent in fine powders and, on the other hand, to the presence of passivating layers at the surface of the grains which limit the transport of the active materials to the interfaces.
To overcome this problem it has been proposed to change the composition of the materials of the anodes by adding various additives which will appreciably modify the behavior of the composite electrodes as a source and receiver of lithium.
Various methods have been proposed to improve the operation of the lithium electrodes, such as lithium-aluminium in a salt melt or in an organic medium. These methods, described in U.S. Pat. No. 4,158,720 (negative electrode consisting of lithium-aluminium-iron), U.S. Pat. No. 4,130,500 (negative electrode consisting of lithium-aluminium-magnesium), U.S. Pat. No. 4,002,492 (anode consisting of an agglomerated alloy of lithium-aluminium) and in U.S. Pat. No. 3,957,532 (anode obtained from an alloy of lithium-aluminium containing a metallic mesh) do not seem to overcome all of the difficulties mentioned above.
More recently, in a study relating to various additives for improving the loss of capacity of the cycling of lithium-aluminium electrodes in molten salt cells, such as in a LiCl-KCl medium, T. D. Kaun and William G. Reder (The Electrochem, Soc., Spring Symp., May 9-14 1982, Montreal, Quebec, Canada, Abstract No. 345) mention the beneficial effect, in some cases, of adding 3 to 5% of an inert charge, with respect to the volume of the electrode, said charge being either a semi-conductor, carbon, an insulating material, MgO, or a metallic conductor, AlFe, all of these in powder form of about 75 .mu.m in diameter.
The improvements to the cycling are substantial, but there is little or nothing to be gained on the utilization. On the other hand, improved performances seem to be connected, according to Kaun and Reder, to the absence of sintering of the lithium-aluminium powders when carbon, MgO or AlFe are added thereto. The electrodes of Kaun and Reder are different from the electrodes according to the present invention both in their formulation and in the experimental conditions under which they can be used: molten electrolyte (400.degree.-500.degree. C.), sintering or agglomeration of lithium-aluminium during cycling, different proportions, etc.
At low temperature, as is the case when the electrodes according to the present invention are used, it is obvious that there are not sintering problems, or at least, if there are some, they are not significant. Consequently, the addition of carbon in the case of Kaun et al does not play an equivalent role to that of the electrodes according to the present invention, because with molten salts, the substitution of carbon by MgO produces the same beneficial effect.
On the other hand, preliminary tests on the addition of MgO powder to the material of the electrode have been found to be without effect on the use of the materials of the electrode according to the present invention. Similarly, the addition of electronic conducting material such as powdered nickel or copper, does not result in any interesting improvement with respect to the use of lithium-aluminium. At the most, these additions permit to increase the use of lithium-aluminium to 5-6%, while it is normally of the order of 3% without additives. This improvement can easily be related to a better conductivity of the material.
It has been realized according to the present invention that it is possible to overcome the problem of the use of lithium or other alkaline metals, such as sodium resulting from the use of fine powders which are prepared as thin films (&lt;50 .mu.m) by adding to the composition of the electrode particles of carbon, such as graphite or blacks, for example acetylene blacks which contain little impurities capable of reacting with the lithium alloys. A good description and definition of the blacks will be found in "Kirk-Othmer Encyclopedia of Chemical Technology", Vol. 4, pp. 243-282, Second Edition, 1967, John Wiley & Sons. In this manner, the use of lithium is substantially increased (&gt;50%) without substantially reducing the potential of the cell or of the added material. The mechanism which explains this improvement is not well understood but would seem to imply more than a mere improvement of the electronic conductivity of the material of the electrode. The formation of a compound of insertion Li&lt;C&gt; could for example be one of the reasons for the effect which is observed.
The inventors have noted that under the conditions of the preparation of the anode according to the present invention, the blacks could insert the lithium at potentials lower than +1.2 volts with respect to a lithium electrode. They have also noted that under these conditions, the total coefficient of diffusion of the lithium in the anode is then improved.
The noted increase of the electronic conductivity of the materials due to the presence of metal powders, grahite or blacks, is superior in the case of the addition of blacks, even at lower volume contents, because of their large dispersion and/or specific area. It has also been noted that the blacks enable to improve by a factor of five and even by a factor of ten, in certain cases, the use of the lithium in the material of the electrode.
Even though the use of additives such as blacks is well known for positive electrodes, in order to improve the collection of current and the distribution of the potential, this necessity does not generally arise on the side of the negative electrodes when metals or alloys are used. These metals or alloys possess sufficient electronic conductivity and are generally compacted or are bound by pressing or sintering. This is for example the case when an alloy of lithium-aluminium is used in molten salt batteries (U.S. Pat. No. 3,445,288) or in an organic liquid (U.S. Pat. No. 4,002,492) where the electrodes are generally thick and rigid and consequently not very adaptable to the concept of thin film batteries.
On the other hand, taking into account the possible reaction between the lithium and the carbon additive under the conditions according to the present invention, it is obvious that the use of a black or a graphite pre-inserted with lithium falls within the scope of the present invention.
The invention is distinguished by the fact that it enables to obtain an electrode based on an alloy of lithium having a large exchange surface which can be prepared in the form of a thin film by using as an electrolyte a polymer which is binding and flexible and makes it possible to maintain the contacts during cycling and absorbs the variations of the volume of the electrodes, said electrode also containing a carbon additive to optimize the use of the active material while enabling the assembly of the composite electrode to retain its plastic and elastomeric properties.